1. Field of the Invention
This invention relates to laser shock peened intermetallic articles of manufacture and methods to laser shock peened intermetallic articles and, more particularly, to gas turbine engine parts such as intermetallic airfoil leading edges having localized compressive residual stresses imparted by laser shock peening.
2. Description of Related Art
Gas turbine engines and other machinery and engines operate parts in high temperature conditions that often require heat resistant metallic parts. Cobalt and nickel-base superalloy materials have been developed to provide mechanical strength at high temperatures so that the operating temperature capability of compressor and turbine blades was increased over the operating temperatures of prior designs but they are heavy. In an effort to reduce the weight of gas turbine engines intermetallic materials have been developed, particularly, for use as compressor and turbine blades. Low pressure turbine blades fabricated from gamma titanium aluminide have successfully been tested and use of such blades could eventually result in reducing the weight of aviation gas turbines by hundreds of pounds. Such blades typically have about half the weight of comparable components made from conventional nickel-based metallic alloys. The tested intermetallic material had 49% titanium, 47% aluminum, 2% chromium, and 2% niobium and when compared with conventional nickel-based alloys, the intermetallic material has half the density and is comparable in strength out to about 1400 degrees Fahrenheit. The titanium aluminide also is about 50% stiffer than conventional titanium alloy blades. It has been estimated that if an intermetallic material was used in the low pressure section of the GE90 as a blade material, the titanium aluminide could cut engine weight by more than 300 pounds. Lighter blades would also allow turbine wheels to be lighter and less robust because the reduced weight blades create lower stresses during operation.
Intermetallic materials vary widely in composition but are generally defined to include materials between metal and ceramic, as described in U.S. Pat. No. 5,299,353 xe2x80x9cTURBINE BLADE AND PROCESS FOR PRODUCING THIS TURBINE BLADExe2x80x9d. Gamma-titanium aluminides used for turbine and compressor blades have properties which are beneficial to their use as a material for turbine blades exposed to high temperatures. These include, among other things, their density, which is low in comparison with superalloys conventionally used, for example where Ni-superalloys are concerned the density is more than twice as high. The material of the turbine blade disclosed in U.S. Pat. No. 5,299,353 has a comparatively high heat resistance, but the ductility of this material at room temperature is comparatively low and, therefore, damage to parts of the turbine blade subjected to bending stress cannot be prevented with certainty.
One major drawback with blades made from intermetallic materials is their much lower ductility and brittleness. It is well known that titanium aluminides and other intermetallic materials are highly difficult to work with because their ductility is only a few percent at room temperature. Blades must be hardened and toughened to prevent failing of substantial portions of the blade due to foreign object damage (FOD). The resultant fatigue failure that may occur is due to the formation and propagation of a crack. Conventional shot peening of metallic articles to impart compressive residual stresses to relatively low depths is well known in the art and has even been suggested for use on thin blade edges in U.S. Pat. No. 4,426,867. However, shot peening has never been even suggested for use on articles made of intermetallic materials because of their well known low ductility and brittleness properties.
The region of compressive residual stresses imparted by laser shock peening of the present invention is not to be confused with a surface layer zone of a metallic work piece that contains locally bounded compressive residual stresses that are induced by a hardening operation using a laser beam to locally heat and, thereby, harden the work piece such as that which is disclosed in U.S. Pat. No. 5,235,838, entitled xe2x80x9cMethod and Apparatus for Truing or Straightening Out of True Work Piecesxe2x80x9d. The present invention uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a work piece made of intermetallic material similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled xe2x80x9cAltering Material Propertiesxe2x80x9d; U.S. Pat. No. 4,401,477, entitled xe2x80x9cLaser Shock Processingxe2x80x9d; and U.S. Pat. No. 5,131,957, entitled xe2x80x9cMaterial Propertiesxe2x80x9d for metallic materials. Laser peening as understood in the art and as used herein means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of the metallic workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled xe2x80x9cLaser Peening System and Methodxe2x80x9d. However, the prior art does not disclose or even suggest laser shock peening of intermetallic materials such as compressor and turbine blade leading and trailing edges of the type claimed by the present patent nor the methods how to produce them. It is to this end that the present invention is directed. The invention is not limited to compressor and turbine blades but is applicable to other parts of the gas turbine engine and other articles of manufacture that use intermetallic materials.
An intermetallic article having an intermetallic body with at least one laser shock peened surface on at least a portion of the intermetallic body and a region having compressive residual stresses imparted by laser shock peening extending into the intermetallic body from the laser shock peened surface. A particular embodiment of the intermettalic article is a gas turbine engine component such as a compresor or turbine blade and wherein the body is an intermetallic airfoil having a leading edge and a trailing edge and a pressure side and a suction side, and at least one laser shock peened surface on at least one side of the airfoil. The laser shock peened surface extends radially along at least a portion of the leading edge and chordwise from the leading edge, and the region having compressive residual stresses imparted by laser shock peening extends into the airfoil from the laser shock peened surface. Preferably both sides of the airfoil are laser shock peened along the leading edge. The trailing edge may also be provided with similar laser shock peened regions having compressive residual stresses imparted by laser shock peening extending into the airfoil from laser shock peened surfaces.
The present invention also includes a method for laser shock peening an intermetallic article as described above and which includes forming a coated surface by covering a laser shock peened surface on at least a portion of the intermetallic body of the intermetallic article with an ablative medium, providing a confining medium over the coated surface, and firing a laser beam on the coated surface with sufficient power to vaporize the ablative medium to form a region in the intermetallic body having compressive residual stresses imparted by the laser beam pulsing such that the region extends into the intermetallic body from the laser shock peened surface. The step of providing a confining medium over the coated surface may be flowing a clear fluid curtain such as water over the coated surface. Forming a coated surface may be accomplished by coating the laser shock peened surface with an ablative paint or adhesively covering the laser shock peened surface with a tape having an ablative medium.
The laser beam, which repeatably pulses between relatively constant periods, may be continuously fired on the coated surface of the body while providing continuous movement between the laser beam and the intermetallic body while firing the laser beam with sufficient power to vaporize the ablative medium with the pulses and forming laser beam spots on the coated surface. The laser may be fired to form a pattern of overlapping laser beam spots while the relative movement is being provided. The intermetallic body may be constructed of a titanium aluminide material suitable for use as a turbine airfoil or, in another embodiment, of a nickel aluminide material suitable for use as a compressor airfoil.
Among the advantages provided by the present invention is the improved ability to use inter-metallic materials in articles that operate in environments that are subject to high tensile and vibratory stress fields and/or which are subject to fatigue failure. The major advantage of using inter-metallic materials is the large weight savings it affords to designs of gas turbine engines and other apparatus. The invention allows the use of these low weight, high strength, and heat resistant materials to be used for gas turbine engines such as for compressor and turbine blades which may be subject to nicks and tears in the leading and trailing edges of the blade and vane airfoils. The present invention provides for an increased life over previously proposed inter-metallic articles and in particular gas turbine airfoil. Another advantage of the present invention is that compressor and turbine blade airfoils can be constructed with commercially acceptable life spans with a reduction in weight of now only the blades themselves but also the weight of the disk or drum rotors that support the blades.